This invention relates to a device for controlling the rotational speed of an electric motor and more particularly to such a device having a chopper circuit equipped with a bypass contactor.
In such a motor speed control device, the bypass contactor is connected in parallel with the chopper circuit. By turning the bypass contactor on in a predetermined manner, the voltage at the battery is directly applied to the electric motor to drive the motor at full speed. This type of control device is widely used in, for example, electric cars. A typical example of such control device is disclosed in U.S. Patent Application Ser. No. 382,804 of W. Heyman et al. entitled "Adjustable speed motor control" filed on July 15, 1964 and issued as U.S. Pat. No. 3,337,786 on Aug. 22, 1967.
In this conventional motor speed control device, the bypass contactor is turned on in a certain constant time after the switch for detecting the fully actuated position or state of the accelerator has been turned on by depressing the accelerator pedal to the full extent. Accordingly, when the driver depresses the accelerator suddenly, the bypass contactor may be turned on even though the duty ratio of the chopper is prevented from increasing due to the current limiting characteristic. Consequently, the duty ratio varies discontinuously so that shocks sometimes occur. Especially in case where an electric car is suddenly started with full load, the car is accelerated very slowly and such a shock is so serious that the goods may be broken.